A gas-insulated device is filled with insulating gas such as SF6 gas. In a conventional water-concentration detection device present in SF6 gas, a water sensor that detects water is installed in a gas-insulated device. This water sensor is configured to include porous electrodes provided to face each other and a hydrogen-ion conductive solid electrolyte membrane that is provided between the porous electrodes and in equilibrium with the water concentration of SF6 gas. This water-concentration detection device measures a water concentration of SF6 gas by applying an alternating-current (AC) voltage to the porous electrodes and measuring inter-electrode AC impedance changing correspondingly to the water concentration of the SF6 gas (see Patent Literature 1).